Mesoporous Mn-substituted MnxZn1−xCo2O4 ternary spinel microspheres with enhanced electrochemical performance for supercapacitor applications

Extensive investigations have been carried out on spinel mixed transition metal oxide-based materials for high-performance electrochemical energy storage applications. In this study, mesoporous Mn-substituted MnxZn1−xCo2O4 (ZMC) ternary oxide microspheres (x = 0, 0.3, 0.5, 0.7, and 1) were fabricated as electrode materials for supercapacitors through a facile coprecipitation method. Electron microscopy analysis revealed the formation of microspheres comprising interconnected aggregates of nanoparticles. Furthermore, the substitution of Mn into ZnCo2O4 significantly improved the surface area of the synthesized samples. The electrochemical test results demonstrate that the ZMC3 oxide microspheres with an optimal Mn substitution exhibited enhanced performance, displaying the largest specific capacitance of 589.9 F g−1 at 1 A g−1. Additionally, the ZMC3 electrode maintained a capacitance retention of 92.1% after 1000 cycles and exhibited a significant rate capability at a current density of 10 A g−1. This improved performance can be ascribed to the synergistic effects of multiple metals resulting from Mn substitution, along with an increase in the surface area, which tailors the redox behavior of ZnCo2O4 (ZC) and facilitates charge transfer. These findings indicate that the incorporation of Mn into mixed transition metal oxides holds promise as an effective strategy for designing high-performance electrodes for energy storage applications.

www.nature.com/scientificreports/Consequently, significant efforts have been devoted to the development of transition metal oxides with tunable nanostructures and compositions to improve their electrochemical performances [11][12][13] .
In recent years, spinel-type multimetal oxides have been extensively investigated as high-performance electrode materials for supercapacitors 14,15 .This is due to the multiple oxidation states, better electrochemical performance, and improved electrical conductivity compared to their single-metal counterparts [16][17][18] .Particularly, spinel cobaltites belonging to the MCo 2 O 4 class with a single-phase structure have been extensively investigated as electrode materials owing to their enhanced electrochemical performance 19,20 .What sets cobalt-based metal oxides apart from other spinel-type materials is their distinctive battery-like behavior, primarily attributed to the generation of oxyhydroxides during charge storage, resulting in a notably high theoretical specific capacity 21 .Furthermore, cobalt exhibits remarkable features by enhancing the electronic conductivity of metal oxides, making it an appealing choice for such applications.The combination of different cations to form multimetallic spinel oxides and the ability to adjust the stoichiometric/non-stoichiometric compositions of these oxides offer great opportunities to manipulate their electrochemical properties [22][23][24] .Thus, strategies based on the substitution of metal cations in host metal oxides and the rational design of nanostructures and microstructures have been widely employed to develop high-performance mixed-transition-metal oxide electrodes.In this regard, numerous studies of transition metal oxides composed of multiple metals, including Mn 1−x Ni x Co 2 O 4 25 , Zn-Mn-Co ternary oxide nanoneedles 26 , Zn-Ni-Co ternary oxide microspheres 27 , Cu-Zn-Co oxide nanoflakes 28 , Mn-doped NiCo 2 O 4 29 , mesoporous MnZnFe 2 O 4 nanoneedles 30 , and mixed ternary transition metal ferrites 31 have been reported.These investigations highlight the synergistic effect of incorporating multiple transition metals and the tunability of their compositions to enhance the electrochemical performance of transition metal oxides as supercapacitor electrodes.Therefore, metal cation substitution is a feasible approach for tuning the chemical composition, electronic structure, electrical conductivity, and electrochemical properties of the electrode materials.
Manganese is regarded as a desirable metal for substitution or doping in host transition metal oxide electrode materials because of its favorable characteristics such as multiple oxidation states, abundance in the Earth's crust, and environmental friendliness [32][33][34] .In most Mn-containing electrode materials, Mn 2+ / Mn 3+ /Mn 4+ is observed, which is responsible for the enhanced carrier mobility, improved specific capacitance and cycling stability, and adjustable surface electrolyte ion adsorption/desorption energy 29,[35][36][37][38][39] .Considering these advantages, numerous studies have focused on the substitution or doping of Mn ions into transition metal oxides to achieve improved electrochemical performance.For example, Sharif et al. 40 synthesized Ni 1−x Mn x Fe 2 O 4 nanoparticles and investigated the effect of the incremental substitution of Ni with Mn on the electrochemical performance of supercapacitor electrodes.They found that the specific capacitance of the prepared Ni 1−x Mn x Fe 2 O 4 nanoparticles increased with an increase in the Mn content.Mary et al. 41 reported Mn-doped ZnCo 2 O 4 with an optimal 10 wt% Mn doping and intact structure and showcased an improved specific capacitance.In another study, Zhang et al. prepared a series of Mn-substituted NiCo 2 O 4 (MNCO) nanowires using a hydrothermal annealing strategy and varying the Mn/Ni molar ratio 33 .The results indicate that the Mn 3+ substituting of Ni 3+ in NiCo 2 O 4 optimizes the electronic structure, enhances the charge transfer kinetics, and improves the electrochemical activity of NiCo 2 O 4 .Recently, Sun et al. 42 reported Mn in situ isomorphism-doped CoCo 2 O 4 porous nanowires prepared using a microwave-assisted hydrothermal process.The Mn-incorporated porous nanowires exhibited an improved capacity and a broadened voltage window.The Mn 2+ and Mn 3+ dopants in CoCo 2 O 4 provide additional redox sites by forming reversible Mn 4+ , thereby enhancing the energy storage capacity.Therefore, Mn doping or substitution in transition metal oxides can be considered an effective strategy to improve their electrochemical performance for supercapacitor applications.
In this study, we prepared ternary spinel microspheres of Mn-substituted Mn x Zn 1−x Co 2 O 4 (ZMC) through a straightforward coprecipitation route followed by calcination based on our previously reported study 43,44 .Our investigation is targeted on the impact of manganese (Mn) substitution on both the structural and morphological characteristics of these microspheres, as well as their electrochemical performance.On substitution of Mn, a significant change is observed on the size of the ZMC microspheres.Specifically, their size grew, and their specific surface area expanded, reaching a remarkable 56.27 m 2 g −1 for ZMC3, surpassing that of ZC (17.79 m 2 g −1 ).The electrochemical evaluations revealed that at the optimal Mn substitution, the porous ZMC3 microspheres achieved an outstanding specific capacitance of 589.9 F g −1 at a current density of 1 A g −1 .Furthermore, they exhibited improved cycling stability, retaining 92.1% of the initial specific capacitance after 1000 cycles, outperforming other compositions.These findings demonstrate the effectiveness of Mn substitution as a viable strategy for enhancing the electrochemical performance of transition metal oxide electrodes for energy-storage applications.Furthermore, this work contributes to the expanding family of ternary spinel oxides among multimetal oxide nanostructures.

Experimental Synthesis of the mesoporous Mn x Zn 1−x Co 2 O 4 microspheres
All chemicals used in this study were of analytical grade and were employed without any additional purification steps.The materials were fabricated using the coprecipitation method as previously reported by our research group, with minor adjustments 41 .In this procedure, each sample was created by dissolving the requisite quantities of metal acetate precursors, specifically zinc acetate, manganese acetate, and cobalt acetates, in a mixture of distilled water and ethanol (in a 10:1 ratio), resulting in a total volume of 230 mL.For instance, to synthesize Mn 0.5 Zn 0.5 Co 2 O 4 microspheres (x = 0.5), 1 mmol of Mn(Ac) 2 •4H 2 O, 1 mmol of Zn(Ac) 2 •2H 2 O, and 4 mmol of Co(Ac) 2 •4H 2 O were dissolved in a mixture of 21 mL of ethanol and 210 mL of distilled water under vigorous stirring.Another solution was prepared by dissolving 60 mmol of NH 4 HCO 3 in 230 mL of distilled water, followed by slow addition to the metal precursor solution with continuous stirring.The resulting mixture was heated to 45 °C and held at that temperature for 9 h with continuous stirring.A pale pink precipitate formed, which was

Characterization of the samples
X-ray diffraction (XRD) was obtained using a Bruker D2 Phaser X-ray diffractometer equipped with Cu K radiation (λ = 0.709321 Å) at 30 kV.The scan spanned from 10° to 70° 2θ in increments of 0.0260°.Transmission electron microscopy (TEM) analysis was conducted utilizing a JEM-2100 transmission electron microscope, while scanning electron microscopy (SEM) analysis employed a FEI Nova Nano SEM 450 scanning electron microscope.The N 2 adsorption/desorption experiments were performed to determine BET surface area and pore size distribution using a Micromeritics ASAP 2020 surface area and porosity analyzer.Prior to the experiment, the sample was subjected to outgassing at 150 °C for 5 h under a nitrogen gas atmosphere.BET surface areas were determined from adsorption data within a relative pressure range of 0.05-0.30,and the total pore volume was calculated based on the quantity of N2 vapor adsorbed at a relative pressure of 0.99.The pore size distributions were derived from the desorption branches of the isotherms.

Fabrication of electrodes and electrochemical performance test
The electrodes were fabricated as follows: the active material, activated carbon, and polyvinylidene fluoride (PVDF) binder were mixed in an 80:10:10 (wt.%) ratio.A mixture slurry was made using N-Methyl-2-pyrrolidone (NMP), which was coated on a nickel foil current collector of (1 × 1 cm) 1.6 mm thickness (produced by MTI corporation) and dried for 4 h to remove the solvent.The weight of active material loaded was in the range of 1-1.5 mg.Cyclic Voltammetry (CV), chronopotentiometry (CP), and Electrochemical Impedance spectroscopy (EIS) were performed using a Gamry potentiostat/galvanostat, employing a standard 3-electrode cell configuration with a platinum wire as the counter electrode and Ag/AgCl as the reference electrode.The measurements were performed using an aqueous 2 M KOH electrolyte under ambient conditions.

Structure and morphology characterizations
Mesoporous Mn x Zn 1−x Co 2 O 4 (ZMC) ternary oxide microspheres were synthesized via a two-step process, as depicted in Scheme 1.Initially, ZMC carbonate microspheres were formed through a simple coprecipitation method using NH 4 HCO 3 as the precipitating agent.Subsequently, the ZMC carbonate microspheres were calcined at 600 °C, resulting in the formation of the ZMC oxide microspheres.This calcination process facilitated the creation of numerous pores by eliminating gaseous H 2 O and CO 2 and promoting interactions between the constituent nanoparticles, thereby resulting in the formation of porous microspheres.To examine the crystalline structure, X-ray diffraction (XRD) was performed in the range of 10°-70° 2θ, as shown in The peaks of the Mn substituted samples are also observed to broaden with increasing Mn concentration compared to the ZnCo 2 O 4 .The Rietveld analysis was utilized to calculate the lattice parameters.It was observed that ZnCo 2 O 4 exhibits a lattice parameter of a = 8.0874 Å, which closely aligns with the reported values.When Mn ions were introduced as substitutes for Zn 2+ , the lattice parameter increased in comparison to pure ZnCo 2 O 4 due to the slightly larger ionic size of Mn ions (0.740 Å) 45 .This increase in lattice parameter demonstrates a clear and linear trend with the gradual doping of Mn ions, confirming the successful substitution of Zn by Mn ions.Additionally, the crystallite size is observed to decrease from 51 to 19 nm with increasing Mn concentration.
The morphology and microstructure of the Mn x Zn 1−x Co 2 O 4 microspheres were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM).The SEM images in Fig. 2 depict ZMC oxide microspheres with diameters ranging from 200 to 600 nm.With the substitution of Mn in ZnCo 2 O 4 , more regular and abundant microspheres tended to form. Figure 3a presents a TEM image of the ZMC3 microspheres, which shows their smooth surface and micron size.In Fig. 3b, the high-resolution TEM (HRTEM) image zooms Scheme 1. Schematic illustration of the formation process of ZMC oxide microspheres.on the edge of the porous microsphere, revealing voids between the primary particles and a substantial quantity of nanoparticles.This porous structure promotes a relatively large contact area between the active material and the electrolyte, facilitating efficient ion transport 46 .Figure 3c shows the selected-area electron diffraction (SAED) pattern, which confirms the polycrystalline nature of the porous ZMC microspheres.This pattern also highlights the presence of abundant nanoparticles with a high phase purity.The EDS spectrum of ZMC3 in Fig. 3d indicates the presence of the Zn, Co, and Mn elements, with the Zn:Mn:Co ratio of 0.3, 0.65 and 2.2, respectively.
The performance of a supercapacitor electrode is influenced by important parameters, such as the surface area and pore size distribution.To understand the textural properties of the as-prepared materials, nitrogen adsorption-desorption analysis was carried out and the values are summarized in Table 1. Figure 4a illustrates a type IV isotherm with a distinct hysteresis loop is observed at a relative pressure of 0.7-1 for all the Mn substituted ZMC samples.This indicates the existence of a mesoporous structure in the ZMC microspheres, which can improve the contact between the electrolyte and electrode and potentially reduce the diffusion paths for electrolyte ions.Incorporating Mn into ZnCo 2 O 4 results in an increase in the surface area and pore volume.Specifically, the ZMC3 oxide microspheres displayed the highest specific surface area (56.27 m 2 g −1 ) and pore volume of 0.29 cm 3 g −1 .This can be attributed to the open pores formed by the nanoparticles within the ZMC3  microspheres, as evidenced by the TEM results shown in Fig. 3b.Hence, it is anticipated that ZMC3 will exhibit superior electrochemical activity because of its large surface area and optimal pore size.The pore size distributions were determined by analyzing the desorption section of the isotherms, as shown in Fig. 4b.The results together with the summary of the values in Table 1 clearly demonstrated the presence of numerous mesopores ranging in size from 10 to 50 nm, indicating the simultaneous existence of structural and interconnected pores.www.nature.com/scientificreports/Moreover, the pore size distribution curve revealed a prominent peak at approximately 23 nm, indicating a distinct mesoporous structure.BET analysis results confirmed that the substitution of Mn into ZnCo 2 O 4 significantly enhanced the surface area, thereby contributes to an improved electrochemical performance of the electrode materials.

Electrochemical performance
The evaluation of the supercapacitive performance of the Mn x Zn 1−x Co 2 O 4 (ZMC) electrodes was performed by voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements.All measurements were conducted in 2 M KOH electrolyte.Figure 5a displays the cyclic voltammetry (CV) curves of the ZMC oxide electrodes in a standard 3-electrode configuration utilizing the fabricated electrode as the working electrode, a platinum wire as the counter electrode, and Ag/AgCl as the reference electrode.CV was carried out at a scan rate of 50 mV s −1 within a potential range of − 0.1 − 0.55 V.The CV curves of all samples consist of pairs of well-defined redox peaks, indicating the presence of Faradaic redox reactions.This is quite different from the rectangular-shaped curves of the electric double-layer capacitance (EDLC), suggesting the establishment of the pseudocapacitive charge storage mechanism 47 .Among the samples, the ZMC3 sample displayed the highest redox peak current and largest integrated CV area, which can be attributed to the addition of manganese ions into the ZnCo 2 O 4 spinel lattice 48,49 .Figure 5b shows the CV curves of ZMC3 at different scan rates.The shape of the CV curves remained consistent as the scan rate increased, indicating electrochemical reversibility and highrate performance.However, there is a shift in the redox peaks towards lower potentials, which may be due to the polarization effect of the electrodes 50 .These redox peaks, originating from the Faradaic redox reactions, can be associated with M-O/M-O-OH (M = Mn, Zn, Co).The pair of redox peaks in the CV curves can be explained using the following equations 39,51,52 : To further investigate the potential applications of the porous ZMC oxide microspheres as electrode materials for supercapacitors, we conducted galvanostatic charge discharge (GCD) measurementsat different current densities.Figure 5c illustrates the comparative galvanostatic discharge curves of the ZC, ZMC1, ZMC2, ZMC3, and MC electrodes at a current density of 1 A g −1 in the potential range of 0 to 0.45 V.The discharge curves show the pseudocapacitive characteristics of the as-prepared ZMC electrodes displaying a non-linearity and plateau (1) regions.These features demonstrate the presence of Faradaic redox processes, and they agree well with redox peaks observed in the CV curves.A previous study on the Mn-substituted Mn x Zn 1−x Co 2 O 4 oxides carried out by our group utilized XPS analysis and revealed the presence of Mn 2+ , Mn 3+ , Co 2+ , and Co 3+ in all compositions, which are important for redox reactions 44 .Consequently, the pseudocapacitive behavior observed in the electrode materials can be ascribed to the presence of Co 2+ /Co 3+ and Mn 2+ /Mn 3+ redox couples.This observation aligns well with the existing literature, which emphasizes the coexistence of Mn 2+ and Mn 3+ upon Mn doping, thereby enhancing the electrochemical performance of the host transition metal oxide electrode 42,48 .Moreover, looking more in to the GCD curve, it was observed that the ZMC3 exhibits a longer discharge time compared to other samples at the same current density and the same potential window.This indicated that the ZMC3 electrode exhibited a higher specific capacitance.For a more comprehensive understanding, the charge/discharge curves of ZMC3 at various current densities (1-10 A g −1 ) are presented in Fig. 5d.The specific capacitances C s (F g −1 ) of the electrodes were calculated from the discharge curve using the following equation: where I (mA) is the applied current, Δt (s) is the total discharge time, m (mg) is the mass of electroactive material, and ΔV (V) is the potential drop during discharge.According to Eq. ( 3), the specific capacitance of the ZMC3 microspheres was determined to be 589.9F g −1 at 1 A g −1 .This value surpassed those of ZC (246.4F g −1 ), ZMC1 (306 F g −1 ), ZMC2 (342.6 F g −1 ) and MC (286.7 F g −1 ) samples at the same current density.The increased specific capacitance observed in the ZMC3 sample can be attributed to its higher surface area and additional redox sites.These factors promote better interaction between the electrolyte and the active electrode material, facilitating more efficient electron transport.This electrochemical performance is comparable to that reported in the literature (Table 2).Mary et al. 41 reported hydrothermally synthesized Mn-doped ZnCo 2 O 4 nanoparticles with 1, 2, 5, 10, and 15% Mn doping by weight.The 10 wt% Mn-doped ZnCo 2 O 4 sample achieved a maximum capacitance of 707.4 F g −1 at a current density of 0.5 A g −1 .The presence of Zn, Mn, and Co in the Mn-substituted samples synergistically enhanced their electrochemical performances.In another study, Singh and colleagues explored the effect of Mn doping on the energy storage properties of Mn doped spinel FeCo 2 O 4 (FeMn x Co 2−x O 4 ) nanofibers.The results demonstrated that Mn doping improved both the magnetic and energy storage properties of the FeCo 2 O 4 spinel oxide, attaining a specific capacitance of 212 F g −1 at 3 mV s −1 and a Mn content of x = 0.2.
Supercapacitor electrode materials require cycling stability, which is a crucial factor for evaluating and comparing their performance with those of other reported materials.To evaluate the stability of the Mn-substituted ZMC1, ZMC2, and ZMC3 ternary oxide microspheres, they were subjected to 1000 charge-discharge cycles at a constant current density of 5 A g −1 in a 2 M KOH solution.The results are shown in Fig. 6a.The figure clearly demonstrates that the ZMC3 sample displays superior cyclic performance compared to the Mn-substituted ZMC1 and ZMC2 samples.After 1000 cycles, the electrode retained a specific capacitance of approximately 519 F g −1 .
In the first 450 cycles, the specific capacity increased from 383 F g −1 and subsequently stabilized, maintaining a stable capacity for up to 1000 cycles.Furthermore, when the current density was increased from 1 to 10 A g −1 (Fig. 6b), the specific capacitance of ZMC3 decreased to 533.1 F g −1 , indicating a capacitance loss of only 7.9%.This result demonstrates the high rate capability at a high current density, which is a very important feature of the electrodes for practical applications.
EIS measurements were carried out to further investigate the electrical conductivity and electrochemical behavior of the electrodes at frequencies ranging from 100 kHz to 0.1 Hz with an amplitude of 10 mV.The Nyquist plots presented in Fig. 6c,d depict the characteristics of the ZC, ZMC1, ZMC2, ZMC3, and MC electrodes along with a magnified view of the high-frequency region.The inset in Fig. 6c shows an equivalent circuit diagram of the electrode materials.A typical EIS plot contains a semicircle in the high-frequency region and a straight line in the low-frequency region.The high-frequency region reflects the charge-transfer resistance (R ct ) produced by the redox reactions that occur on the surface of the electrocatalyst.The low-frequency behavior reflects the diffusion of the electrolyte 59 .The Mn substituted Mn x Zn 1−x Co 2 O 4 electrodes exhibit equivalent series resistance (ESR) in the range of 0.96-1.1 Ω with ZMC3 providing relatively low overall resistance.The low ESR and charge transfer resistance for ZMC3 sample reveals its relatively high conductivity.The other major factor influencing the electrochemical performance of these materials is the alteration of the intrinsic activity of Mnsubstituted ZnCo 2 O 4 .
(3) As discussed in the preceding analysis, the enhanced electrochemical characteristics of the ZMC3 sample can be predominantly attributed to the synergistic influence of various multimetal compositions and the notably increased specific surface area achieved through Mn substitution 60 .The formation of mesoporous microspheres offers a high surface area accessible to electrolyte ions and enables faster kinetics and higher utilization of the active material.The substitution of Mn for Zn is believed to modify the distribution of cations across the tetrahedral and octahedral sites in the ternary spinel oxides, thereby fine-tuning their electrochemical properties and providing abundant electroactive sites 44 .In addition, the existence of various Mn oxidation states enhances the redox behavior of ZnCo 2 O 4 by providing rich redox sites.While these results hold promise, a more comprehensive characterization and extensive electrochemical assessments utilizing two-electrode systems are essential to fully comprehend the intrinsic properties of these electrode materials and their potential for practical applications, which forms the next phase of this study.

Conclusion
In summary, we prepared mesoporous Mn-substituted Mn x Zn 1−x Co 2 O 4 (x = 0, 0.3, 0.5, 0.7, 1) ternary oxide microspheres by employing a simple coprecipitation method followed by calcination.We systematically investigated the effects of Mn substitution on the structural and morphological evolution as well as the electrochemical performance of these materials.The XRD results confirmed the successful formation of spinel Mn-substituted Mn x Zn 1−x Co 2 O 4 ternary oxides, while the TEM and SEM images indicated the formation of microspheres with an enhanced specific surface area, providing more active sites for electron transport.The electrochemical activity of Mn x Zn 1−x Co 2 O 4 ternary oxides, when tested as an electrode material for supercapacitors, achieved optimum performance at a Mn concentration of x = 0.7.Notably, the ZMC3 electrode displayed the highest specific capacitance of 589.9 F g −1 at 1 A g −1 and retains 92.1% of its initial specific capacitance after 1000 cycles.Moreover, ZMC3 showed high capacity retention when tested at current density of 1-10 A g −1 indicating their remarkable rate capability.These findings highlight the significant improvement in the specific capacity and rate capability of Mn-substituted Mn x Zn 1−x Co 2 O 4 oxide microsphere electrodes achieved through Mn doping.Therefore, this study demonstrates the effectiveness of tuning the composition of multimetal transition metal oxides to significantly improve their electrochemical performance.
Fig. 1.The XRD patterns of all the samples show the main characteristic peaks at 18.5°, 31.3°,36.2°,44.3°, 59.1°, and 64.3°, which are well indexed to the (111), (220), (311), (422), (511), and (440) crystallographic planes, respectively.The XRD patterns were assigned to ZnCo 2 O 4 (JCPDS no.23-1390) and/or MnCo 2 O 4 (JCPDS no.23-1237), both with a cubic spinel structure and crystallizing in the Fd-3 m space group (227).No discernible diffraction peaks originating from impurities are observed in any of the XRD patterns.These results confirmed the successful formation of ternary spinel ZMC oxides.In comparison to the pure ZnCo 2 O 4 , the most intense peak of the (311) plane in Mn x Zn 1−x Co 2 O 4 displayed a slight shift towards a lower angle.This shift indicated the successful incorporation of Mn atoms into the crystal lattice of ZnCo 2 O 4 through doping.

Figure 5 .
Figure 5. (a) Cyclic voltammetry (CV) curves of ZC, ZMC1, ZMC2, ZMC3, and MC at a scan rate of 50 mV s −1 in 2 M KOH electrolyte.(b) CV measurements of the ZMC3 sample at different scan rates from 5 to 100 mV s −1 ; (c) galvanostatic discharge curves of the ZC, ZMC1, ZMC2, ZMC3, and MC samples at a current density of 1 A g −1 ; (d) galvanostatic charge/discharge curves of the ZMC3 sample at different current densities.

Figure 6 .
Figure 6.(a) Variation of specific capacitance with cycle number for ZMC1, ZMC2, and ZMC3 samples over 1000 cycles at a current density of 5 A g −1 ; (b) specific capacitance of ZMC3 electrode at different current densities; (c) Nyquist plots for ZC, ZMC1, ZMC2, ZMC3, and MC samples with an amplitude of 10 mV in 0.1 Hz to 100 kHz and the magnified portion of the higher frequency region.The inset in (c) shows the circuit used to fit the data.(d) Zoomed section of the Nyquist plots.

Table 2 .
Specific capacitances of ternary metal oxide-based electrode materials for comparison.